Low temperature glazing composition

ABSTRACT

A low temperature glazing composition consisting essentially of a mixture of 25-75% by weight of an aqueous dispersion of an acrylic resin having a monomer composition of 40-85% by weight methyl methacrylate, 45-15% by weight butyl acrylate and 0-15% by weight methacrylic acid, this dispersion having 15-25% non-volatiles and a pH of 8.5-10.5; and 75-25% by weight of a powdered resin composition having a particle size within the range of 100-400 mesh consisting essentially of 100-80% by weight of a resin having a monomer composition of 60-85% by weight methyl methacrylate and 40-15% by weight butyl acrylate having a molecular weight of 10,000-20,000 and a Tg greater than 45°C and 0-20% by weight of a pigment.

This present invention relates to a low temperature glazing compositionfor use in conjunction with home-type arts and crafts materials. Moreparticularly, the present invention relates to a ceramic-like glazingcomposition which forms a glazed finish on ceramic or similar articlesat low temperatures, such as are obtainable in a home oven.

During recent years home arts and crafts have received a great impetus;however, ceramic work as a home hobby has suffered due to the expenserequired in obtaining and utilizing a kiln suitable for firing andglazing ceramic articles.

Recently there has been developed a clay composition which has theattributes and working properties of normal clay but which contains aspecial binder so as to be baked and fired in a conventional householdoven at relatively low temperatures. This composition is disclosed in myU.S. Pat. No. 3,817,897.

However, the advantage obtained by having a clay composition which canbe fired and finished in an oven is lost if no suitable method forimparting a ceramic-like glaze to the finished products at similartemperatures is available. It is known to merely paint these products;however, the result achieved is not particularly satisfactory.Accordingly, the glaze compositions of the present invention weredeveloped so as to form ceramic-like fired glazes on both oven-fired andkiln-fired clay products at household oven temperatures in the range ofless than 400°F.

The composition of the present invention comprises 25-75% by weight ofan aqueous acrylic resin dispersion and 75-25% by weight of an acrylicresin powder preferably containing a small percentage by weight of anorganic or inorganic pigment. Also, since some of the glazes of thepresent invention do not impart kiln magic to the final product, it is afurther embodiment of the present invention to utilize an aqueousemulsion polymer in the oven-baked glazing composition which have asufficiently high minimum film-forming temperature so that the resultingfilm, when baked in an oven, becomes discontinuous thereby impartinginteresting and varied effects to the finished ceramic article.

It is, therefore, the primary object of the present invention to providea ceramic glazing composition suitable for home use.

It is a further object of the present invention to provide a ceramicglazing composition which forms a hard, glazed finish at temperaturesattainable in a home oven.

It is a further object of the present invention to provide a ceramicglazing compound which, when baked in a home oven, produces a certaindegree of film discontinuities to produce varied and interesting effectson the final product.

It is a still further object of the present invention to provide amethod for decorating ceramic articles at relatively low temperatures.

Still further objects and advantages of the composition of the presentinvention will become more apparent from the following more detaileddescription thereof.

The present invention relates to a low temperature ceramic glazingcomposition consisting essentially of a mixture of 25-75% by weight ofan aqueous dispersion of an acrylic resin having a monomer compositionof 40-85% by weight methyl methacrylate, 45-15% by weight butyl acrylateand 15-0% by weight methacrylic acid, said dispersion having 15-25% byweight non-volatiles and a pH within the range of 8.5-10.5; and 75-25%by weight of a powdered resin composition having a particle size ofbetween 100-400 mesh consisting essentially of 100-80% by weight of aresin having monomer composition of 60-85% by weight methyl methacrylateand 40-15% by weight butyl acrylate having a molecular weight of10,000-20,000 and a Tg greater than 45°C and 0-20% by weight of apigment.

The method of the present application comprises applying to aceramic-like article at least one coating of a composition consistingessentially of an aqueous dispersion of a priming acrylic resin having15-25% non-volatiles, the resin having a monomer composition of 40-85%by weight of methyl methacrylate, 45-15% by weight butyl acrylate and0-15% by weight methacrylic acid at a pH of 8.5-10.5, mixed with 0-10%by weight pigment; optionally baking the glazed article at 250°-400°Ffor from 5 to 60 minutes; applying at least one coating of a mixture ofaqueous dispersion and powdered resin to the primed ceramic article andbaking the article to a temperature within the range of 250°-400°F forfrom 30 minutes to 4 hours.

As used in the present invention, the term ceramic means either a truehigh fired ceramic article, a ceramic-like article such as disclosed inU.S. Pat. No. 3,817,897, glass or metals.

The glazing composition of the present invention comprises two parts,the first part is a glazing liquid; and the second part is a resinpowder preferably containing pigment.

The glazing liquid is essentially an aqueous dispersion of an acrylicresin in either solution or emulsion form. This acrylic resin has amonomer composition of 40-85% by weight methyl methacrylate, 45-15% byweight butyl acrylate and 15-0% by weight methacrylic acid. the resin isa low molecular weight polymer and has a weight average molecular weightin the range of 5,000-30,000. This resin dispersion contains 15-25% byweight non-volatiles and has a pH of 8.5-10.5.

The MFT (minimum film-forming temperature, which is an indication of thetemperature at which the particular resin solution or emulsion will forma continuous film, is not critical and, if properly used, high and lowMFT materials will produce continuous films. In general, at temperaturesabove the MFT, a particular resin film will be continuous, while, attemperatures below the MFT, this film will become discontinuous and forminternal stresses which create crazing and cracking of the resultantfilm.

If the resin dispersion is a film-former at room temperature, i.e., itmust have an MFT of less than 35°C, no particular pretreatment of theceramic surface is necessary, except for a prime coat. However, if aresin dispersion having an MFT greater than 35°C is used, the surfacemust be primed with the same dispersion and baked for a short time.Further coatings of the disprsion over itself will produce a high glosscontinuous film.

By using resin solutions having from 15-25% non-volatiles at the pH offrom 8.5-10.5, the particular monomer composition of the glazing vehicleof the present invention produces good flow characteristics at elevatedtemperature so as to produce a glazed film having a relatively uniformthickness. Two preferred polymers for producng a continuous glazed filmcomprise a solution polymer of 52% methyl methacrylate, 38% butylacrylate and 10% methacrylic acid and an emulsion polymer of 84% methylmethacrylate and 16% butyl acrylate.

The resin powder portion of the composition of the present inventioncomprises 100-80% of a resin and 0-20% of a pigment and preferably96-80% resin and 4-20% pigment. The resin powder which comprises 25-75%by weight of the entire composition has a particle size within the rangeof 100-400 mesh and it is particularly preferred that the particle sizeis about 200 mesh. This powdered pigment consists essentially of 100-80%and preferably 96-80% by weight of a resin having a monomer compositionof 60-85% by weight methyl methacrylate and 40-15% by weight butylacrylate and has a molecular weight in the range of 10,000-20,000 and aTg greater than 45°c. The preferred resin has a monomer content of 84%methyl methacrylate and 16% butyl acrylate. The pigment, which can beany conventional colored inorganic or organic material, is present inthe range of 0-20% and preferably 4-20% by weight. If it is desired, acolorless coating can be formed by using no pigment; however, if coloris desired, at least 4% by weight should be used.

If a colored coating or glaze is desired, any conventional organic orinorganic pigment or dye which is color fast and stable at temperaturesof 250°-400°F can be used. When used in this specification and claims,pigment means both pigments and dyes. Suitable inorganic pigmentsinclude red iron oxide, yellow iron oxide, i.e., hydrated Fe₂ O₃, carbonblack, titanium dioxide, chromium oxide, etc. Suitable organic pigmentsinclude phthalocyanine blue, hansa yellow toner, naphthol red, toludinered, phthalocyanine green, ultramarine blue, diarylide yellow, etc. alsosome suitable dyes include FD&C yellow No. 5, FD&C yellow No. 6, FD&Cred No. 2, FD&C red No. 4, etc.

The mixture of resin powder and pigment is formed by mixing a smallamount of pigment, preferably in an aqueous dispersion, with a largepercentage of polymer in an aqueous dispersion. This mixture is thenformed into a powder by any conventional method, such as spray drying orair drying followed by grinding into a fine powder.

The composition of the present invention is applied to ceramic articlesby first applying a coating of the resin dispersion alone, or mixed witha small amount of pigment, as a prime coat. This aids in reducing theair holes which may form during baking. If the prime coat has an MFT ofgreater than 35°C, the article must be pre-baked at from 250°-400°F for5 to 60 minutes. Then an aqueous dispersion of a resin, either the sameas the priming resin or different, is mixed in the proper ratio, i.e.,from 3:1 to 1:3, liquid to powder with a powdered resin which preferablywill contain a pigment and applied to the ceramic article by anyconventional means. The MFT of the prime coat and the aqueous dispersionused to mix with the powdered resin should have a similar MFT, i.e., ifthe MFT of the prime coat is above 35°C, the MFT of the aqueousdispersion also should be above 35°C, although if a prime coat having anMFT above 35°C is used an aqueous dispersion with a lower MFT can beused to coat and decorate the article. Although one coating may besufficient, it is often desirable to apply two or more coats of the mixof powdered resin and aqueous resin dispersion before baking to insurean even coating or glaze. This coated article is placed in aconventional home oven at a temperature in the range of 275°-450°F for30 minutes to 4 hours. The preferred baking conditions are a temperatureof 300°F for 2 hours.

A surprising feature of the glazing compositions of the presentinvention is the unusual effects which can be achieved by varying thetype of coating. As noted above, the MFT of the aqueous dispersion isnot critical as long as these compositions with a high MFT, i.e.,non-film formers, are not coated over a previous composition having alow MFT. It has been found that, although these high MFT coatings formsuperior continuous films when applied to a properly primed surface,surprising and highly decorative results are obtained when the high MFTresin, i.e., above 35°C, is applied directly over a previously coatedand baked surface of low MFT coating or over a high MFT coating whichhas been further primed with a coating of a low MFT dispersion.

By varying the number of coatings of continuous glazing material anddiscontinuous glazing material, a variety of differing effects can beproduced.

The coating compositions and methods of the present invention are uniquein that they permit a very high solids level of coating to be placed onthe article to be coated. The mixture of the resin dispersion and resinpowder can have a combined solids content of 40-60% which adheres to thearticle before baking and which flows to form a relatively thickcontinuous or discontinuous film.

The composition and method of the present invention will now beillustrated by the following examples which are for the purposes ofillustration and wherein all parts and percentages are by weight and alltemperatures are in degrees Farenheit.

EXAMPLE 1

The aqueous dispersion resin used as the liquid vehicle is prepared bycharging 49 lbs. of organic solvent, such as toluene, to a reactor andheating to 203°F. Some 1/2 lbs. of t-butyl peroctoate initiator is thenadded, followed by the following monomer mixture added at a constantrate for 60 minutes:

    ______________________________________                                        Methyl methacrylate   25.5    lbs.                                            Butyl acrylate        18.6    lbs.                                            Acrylic acid          4.9     lbs.                                            t-Butyl peroctoate    1.3     lbs.                                            ______________________________________                                    

After the monomer mixture has been added, a further 0.2 lbs. of t-butylperoctoate is added, and the reaction mixture is held at 203°F for 2hours. This mixture is allowed to cool and the solvent is evaporated ina wiped film evaporator at a temperature in the range of 380°F-420°F. Anammonia cut is prepared by adding 8.4 grams of 28% NH₄ OH to 393 gramsof heated (140°F) deionized water. 100 grams of dried resin is added tothis mix with minimal agitation followed by heating for about 30-60minutes until the cut is formed. This material is useful as the primingcompound and as the liquid vehicle for the powdered pigment in theglaze.

A red powdered pigment-resin mix is prepared by spray drying 96% of a30% non-volatile aqueous emulsion of 75% methyl methacrylate and 25%butyl acrylate (Tg=79°C, Mw=14,700) prepared using conventional emulsionpolymerizing action techniques with 2.5% red iron oxide pigment, 0.75%deionized water and 0.75% aqueous ammonia at a temperature of 150°F. anapproximately 1 to 1 mixture of the above powder with the above resincut was made and two coats were applied to a ceramic article which hadbeen previously been coated with the resin cut as a primer. This articlewas then baked at 300°F for 2 hours and produced an even red glazedsurface.

EXAMPLE 2

An emulsion having 75% methyl methacrylate and 25% butyl acrylate wasprepared and mixed with a white powdered titanium dioxide pigmentprepared as in Example 1 in a 1 to 1 ratio. One coat of this mix wasapplied to the glazed article of Example 1 and allowed to dry for 2hours until fine cracks appeared in the white coating. The article wasagain baked at 300°F for 2 hours and produced a finished article havinga decorative red and white appearance.

The foregoing examples are for the purposes of illustration only and thecomposition of the present invention should not be construed as limitedthereto, this composition being fully described in the appended claims.

What is claimed is:
 1. A low temperature ceramic glazing composition consisting essentially of 25-75% by weight of an aqueous dispersion of a first acrylic resin having a monomer composition of 40-85% by weight methyl methacrylate, 45-15% by weight butyl acrylate and 0-15% by weight methacrylic acid, said dispersion having 15-25% non-volatiles and a pH of 8.5-10.5, and 75-25% by weight of a powdered resin composition having a particle size of about 100 to about 400 mesh consisting essentially of 96-80% by weight of a second acrylic resin having a monomer composition of 60-85% by weight methyl methacrylate and 40-15% by weight butyl acrylate having a weight average molecular weight of 10,000-20,000 and a Tg of greater than 45°C and 4-20% by weight of a pigment.
 2. The composition of claim 1 wherein said dispersion is an ammonia cut of a solution polymer of 52% methyl methacrylate, 38% butyl acrylate and 10% methacrylic acid.
 3. The composition of claim 1 wherein said second acrylic resin comprises 84% methyl methacrylate and 16% butyl acrylate.
 4. The composition of claim 2 wherein said second acrylic resin comprises 84% methyl methacrylate and 16% butyl acrylate.
 5. The composition of claim 1 wherein said first acrylic resin has a weight average molecular weight of 5000-30,000.
 6. The composition of claim 1 wherein said powder resin composition has a particle size of about 200 mesh. 